Method for avoiding plume interference in additive manufacturing

ABSTRACT

A method of controlling an additive manufacturing process in which one or more energy beams are used to selectively fuse a powder to form a workpiece, in the presence of one or more plumes generated by interaction of the one or more energy beams with the powder. The method includes controlling at least one of: a trajectory of the one or more plumes, and the one or more energy beams, so as to prevent the one or more energy beams from intersecting the one or more plumes.

BACKGROUND OF THE INVENTION

This invention relates generally to additive manufacturing, and moreparticularly to apparatus and methods for avoiding interference of anenergy beam with an emissions plume in additive manufacturing.

Additive manufacturing is a process in which material is built uplayer-by-layer to form a component. Additive manufacturing is alsoreferred to by terms such as “layered manufacturing,” “reversemachining,” “direct metal laser melting” (DMLM), and “3-D printing”.Such terms are treated as synonyms for purposes of the presentinvention.

One type of additive manufacturing machine is referred to as a “powderbed” machine and includes a build chamber that encloses a mass of powderwhich is selectively fused by a radiant energy beam to form a workpiece.The build chamber is enclosed in a housing that typically includesprovisions for a flow of shielding gas therein. The shielding gas isused to transfer heat away from the surface of the powder bed, toprevent vaporized powder from condensing on the surface of theworkpiece, and to control undesired chemical reactions, such asoxidation.

In operation, the interaction of the radiant energy beam with the powdercauses vaporization of the powder, generating a plume which originatesin the vicinity of the melt pool generated by the energy beam andtravels downstream, entrained in the shielding gas flow. In theimmediate vicinity of the melt pool, the composition of the plume ismostly vaporized powder. At downstream locations, the vapor cools andcondenses so that the plume comprises a mixture of gas and metallicparticles (condensate).

One problem with the presence of the condensate is that it can havedetrimental effects on the build process, for example blockage of theenergy beam, or a reduction in beam intensity. This problem preventsrapid beam scanning or the use of multiple beams.

BRIEF DESCRIPTION OF THE INVENTION

This problem is addressed by a method of using a model or a real-timeunderstanding of the plume behavior and a controller to change the pathof an energy beam being used in an additive manufacturing process sothat it prevents or avoids emissions plumes.

According to one aspect of the technology described herein, a method isprovided of controlling an additive manufacturing process in which oneor more energy beams are used to selectively fuse a powder to form aworkpiece, in the presence of one or more plumes generated byinteraction of the one or more energy beams with the powder. The methodincludes controlling at least one of: a trajectory of the one or moreplumes, and the one or more energy beams, so as to prevent the one ormore energy beams from intersecting the one or more plumes.

According to another aspect of the technology described herein, a methodis provided of controlling an additive manufacturing process in whichone or more energy beams are used to selectively fuse a powder to form aworkpiece, in the presence of one or more plumes generated byinteraction of the one or more energy beams with the powder. The methodincludes controlling at least one of: a trajectory of the one or moreplumes, and the one or more energy beams, so as to prevent the one ormore energy beams from intersecting the one or more plumes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is a schematic, partially-sectioned front elevation view of anexemplary additive manufacturing machine including a build chambertherein;

FIG. 2 is a schematic, partially-sectioned side elevation view of themachine of FIG. 1;

FIG. 3 is a schematic, partially-sectioned top plan view of the machineof FIG. 1;

FIG. 4 is a schematic perspective view of an additive manufacturingbuild chamber showing a build process using two energy beams, with onebeam intersecting a plume generated by the other beam;

FIG. 5 is a schematic perspective view of an additive manufacturingbuild chamber showing a build process using two energy beams, withsensors positioned around the build chamber to detect plumes generatedby the beams;

FIG. 6 is a schematic perspective view of an additive manufacturingbuild chamber showing a build process using two energy beams, whereineach of the beams is steered to avoid a plume generated by other beam;

FIG. 7 is a schematic perspective view of an additive manufacturingbuild chamber showing a build process using two energy beams, with apath of a second energy beam being steered to avoid a plume generated bythe first beam; and

FIG. 8 is a schematic perspective view of an additive manufacturingbuild chamber showing a build process using two energy beams, with a gapcreated in a plume generated by the first beam to permit the second beamto pass therethrough.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 illustratesschematically an additive manufacturing machine 10 suitable for carryingout an additive manufacturing method. Basic components of the machine 10include a table 12, a powder supply 14, a recoater 16, an overflowcontainer 18, a build platform 20 surrounded by a build chamber 22, andat least one beam generator 24, all surrounded by a housing 26. Each ofthese components will be described in more detail below.

The table 12 is a rigid structure defining a planar worksurface 28. Theworksurface 28 is coplanar with and defines a virtual workplane. In theillustrated example it includes a build opening 30 communicating withthe build chamber 22 and exposing the build platform 20, a supplyopening 32 communicating with the powder supply 14, and an overflowopening 34 communicating with the overflow container 18.

The recoater 16 is a rigid, laterally-elongated structure that lies onthe worksurface 28. It is connected to an actuator 36 operable toselectively move the recoater 16 along the worksurface 28. The actuator36 is depicted schematically in FIG. 1, with the understanding devicessuch as pneumatic or hydraulic cylinders, ballscrew or linear electricactuators, and so forth, may be used for this purpose.

The powder supply 14 comprises a supply container 38 underlying andcommunicating with the supply opening 32, and an elevator 40. Theelevator 40 is a plate-like structure that is vertically slidable withinthe supply container 38. It is connected to an actuator 42 operable toselectively move the elevator 40 up or down. The actuator 42 is depictedschematically in FIG. 1, with the understanding that devices such aspneumatic or hydraulic cylinders, ballscrew or linear electricactuators, and so forth, may be used for this purpose. When the elevator40 is lowered, a supply of powder 44 of a desired composition (forexample, metallic, ceramic, polymeric, and/or organic powder) may beloaded into the supply container 38. When the elevator 40 is raised, itexposes the powder 44 above the worksurface 28. Other types of powdersupplies may be used; for example powder may be dropped into the buildchamber 22 by an overhead device (not shown).

The build platform 20 is a plate-like structure that is verticallyslidable below the build opening 30. It is connected to an actuator 46operable to selectively move the build platform 20 up or down. Theactuator 46 is depicted schematically in FIG. 1, with the understandingthat devices such as pneumatic or hydraulic cylinders, ballscrew orlinear electric actuators, and so forth, may be used for this purpose.When the build platform 20 is lowered into the build chamber 22 during abuild process, the build chamber 22 and the build platform 20collectively surround and support a mass of powder 44 along with anycomponents being built. This mass of powder is generally referred to asa “powder bed”, and this specific category of additive manufacturingprocess may be referred to as a “powder bed process”.

The overflow container 18 underlies and communicates with the overflowopening 34, and serves as a repository for excess powder 44.

The apparatus 10 incorporates at least one beam generator 24 operable togenerate an energy beam and direct it as desired. As will be explainedin more detail below, multiple beam generators 24 may be provided andused simultaneously in order to increase this production speed of theapparatus 10. In the illustrated example, two beam generators 24 areshown.

Each beam generator 24 includes a directed energy source 48 and a beamsteering apparatus 50. The directed energy source 48 may comprise anydevice operable to generate a beam of suitable power and other operatingcharacteristics to melt and fuse the powder 44 during the build process,described in more detail below. For example, the directed energy source48 may be a laser. Other directed-energy sources such as electron beamguns are suitable alternatives to a laser.

The beam steering apparatus 50 may include one or more mirrors, prisms,and/or lenses and provided with suitable actuators, and arranged so thata beam from the directed energy source 48 can be focused to a desiredspot size and steered to a desired position in plane coincident with theworksurface 28. For purposes of convenient description, this plane maybe referred to as a X-Y plane, and a direction perpendicular to the X-Yplane is denoted as a Z-direction (X, Y, and Z being three mutuallyperpendicular directions). The beam may be referred to herein as a“build beam”.

In the illustrated example, one of the beam generators 24 is operable togenerate a first build beam 54, and the other of the beam generators 24is operable to generate a second build beam 56.

An exemplary basic build process for a workpiece 25 using the apparatusdescribed above is as follows. The build platform 20 is moved to aninitial high position. The build platform 20 is lowered below theworksurface 28 by a selected layer increment. The layer incrementaffects the speed of the additive manufacturing process and theresolution of the workpiece 25. As an example, the layer increment maybe about 10 to 50 micrometers (0.0003 to 0.002 in.). Powder 44 is thendeposited over the build platform 20 for example, the elevator 40 of thesupply container 38 may be raised to push powder through the supplyopening 32, exposing it above the worksurface 28. The recoater 16 ismoved across the worksurface to spread the raised powder 44 horizontallyover the build platform 20. Any excess powder 44 drops through theoverflow opening 34 into the overflow container 18 as the recoater 16passes from left to right. Subsequently, the recoater 16 may be movedback to a starting position. The leveled powder 44 may be referred to asa “build layer” and the exposed upper surface thereof may be referred toas a “build surface”, designated 45.

One or more of the beam generators 24 are used to melt a two-dimensionalcross-section or layer of the workpiece 25 being built. Within the beamgenerator 24, the directed energy source 48 emits a beam and the beamsteering apparatus 50 is used to steer a focal spot of the build beamover the exposed powder surface in an appropriate pattern. A smallportion of exposed layer of the powder 44 surrounding the focal spot,referred to herein as a “melt pool” is heated by the build beam to atemperature allowing it to sinter or melt, flow, and consolidate. Thisstep may be referred to as “fusing” the powder 44. As an example, themelt pool may be on the order of 100 micrometers (0.004 in.) wide. Inthe illustrated example using two beam generators 24, the first buildbeam 54 generates a first melt pool 58 and the second build beam 56generates a second melt pool 60.

The build platform 20 is moved vertically downward by the layerincrement, and another layer of powder 44 is applied in a similarthickness. The beam generators 24 again emit build beams 54, 56 and thebeam steering apparatus 50 is used to steer the focal spots of the buildbeams 54, 56 over the exposed powder surface in an appropriate pattern.The exposed layer of the powder 44 is heated by the build beams 54, 56to a temperature allowing it to fuse as described above, and consolidateboth within the top layer and with the lower, previously-solidifiedlayer.

This cycle of moving the build platform 20, applying powder 44, and thendirected energy fusing the powder 44 is repeated until the entireworkpiece 25 is complete.

The machine 10 and its operation are as representative example of a“powder bed machine”. It will be understood that the principlesdescribed here are applicable to other configurations of powder bedmachines.

The housing 26 serves to isolate and protect the other components of themachine 10. During the build process described above, the housing 26 isprovided with a flow of an appropriate shielding gas which, among otherfunctions, excludes oxygen from the build environment. To provide thisflow the machine 10 may be coupled to a gas flow apparatus 62, seen inFIG. 2. The exemplary gas flow apparatus 62 includes, in serial fluidflow communication, a variable-speed fan 64, a filter 66, an inlet duct68 communicating with the housing 26, and a return duct 70 communicatingwith the housing 26. All of the components of the gas flow apparatus 62are interconnected with suitable ducting and define a gas flow circuitin combination with the housing 26.

The composition of the gas used may similar to that used as shieldinggas for conventional welding operations. For example, gases such asnitrogen, argon, or mixtures thereof may be used. Any convenient sourceof gas may be used. For example, if the gas is nitrogen, a conventionalnitrogen generator 72 may be connected to the gas flow apparatus 62.Alternatively, the gas could be supplied using one or more pressurizedcylinders 74.

Once the gas flow apparatus 62 and machine 10 are initially purged withgas, the fan 64 is used to recirculate the gas through the gas flowcircuit in a substantially closed loop, so as to maintain positivepressure in the housing 26, with additional added makeup gas added asneeded. Increasing the fan speed increases the velocity and flow rate ofgas in the gas flow circuit; conversely, decreasing the fan speeddecreases the velocity and flow rate of gas in the gas flow circuit. Asan alternative to recirculation, the gas flow apparatus 62 could operatein a total loss mode; for example instead of the gas flowing through thereturn duct 70 and back to the fan 64, it could simply be vented toatmosphere after passing over the build chamber 22. In the illustratedexample, the thermal mass of the gas provides a heat transfer function,however an optional heat exchanger (not shown) could be incorporatedinto the gas flow apparatus 62.

The inlet duct 68 is positioned near the bottom of the housing 26.During operation it provides a stream or flow of gas (see arrow 76). Asseen in FIG. 1, the inlet duct 68 has an elongated shape (for examplerectangular) and discharges gas across the width of the build chamber22. For reference purposes the width of the build chamber 22 may beconsidered parallel to the “X” direction. As shown in FIG. 3, the edgeof the build chamber 22 closest to the inlet duct 68 is referred to as a“leading edge” 78, and the opposite parallel edge is referred to as a“trailing edge” 80. For reference purposes the length of the buildchamber (i.e. distance from leading edge 78 to trailing edge 80) may beconsidered parallel to the “Y” direction.

The gas flow 76 has two functions. First, it is used to effect heattransfer and carry heat away from the surface of the uppermost builtlayer within the build chamber 22. Second, during the build process,some of the powder 44 is vaporized. This vapor can cool and condense onthe surface of the workpiece 25, in turn causing an undesirable surfaceroughness or “recast” layer. Part of the gas flow 76 is used to carryaway the vapors and/or condensate.

In operation, the interaction of the build beams 54, 56 with the powder44 causes heating and vaporization of the powder 44. As shown in FIG. 4,this generates first and second “plumes” 82, 84 respectively whichoriginate in the vicinity of the melt pools 58, 60 and traveldownstream, entrained in the gas flow 76. In the immediate vicinity ofthe melt pools 58, 60 the composition of the plumes 82, 84 respectivelyis mostly vaporized powder. At downstream locations, the vapor can cooland condense so that the plumes 82, 84 comprises a mixture of gas andmetallic particles.

It will be understood that, so long as one of the build beams 54, 56contacts the powder 44 at a location upstream of the other build beam54, 56 relative to gas flow 76, there is a potential for an intersectionof one of the build beams 54, 56 with one of the plume 82, 84. It willfurther be understood that the build beams 54, 56 described abovetypically can be scanned or positioned across the build surface 45faster than the plumes 82, 84 travel, thus creating the potential forthe build beam 54, 56 to intersect its own plume 82, 84.

When one of the build beams 54, 56 intersects a plume 82, 84, thepresence of the condensate can have numerous detrimental effects, forexample blockage of the build beam 54, 56 and/or reduced beam intensity.These effects can be inconsistent because the condensate isscintillating. Accordingly, it is desirable to conduct the build processin such a manner that neither of the build beams 54, 56 passes througheither of the plumes 82, 84. Several techniques for avoiding theseintersections are described below.

To enable the avoidance techniques described elsewhere herein, it isdesirable to quantify the behavior of the plumes 82, 84. In particular,it is desirable to create a “plume map” describing the location anddimensions of each plume 82, 84 in 3-D space for any given time, and thepropagation of the plumes 82, 84 over time. This process may also bedescribed as determining the trajectory of the plumes 82, 84. Severalmethods will be described for characterizing the plumes 82, 84. For thepurposes of convenient description, this will be described using plume82 as an example with the understanding that the same methods may beused for plume 84 or for any additional plume, where multiple energybeams are used.

One possible method for characterizing the plume 82 involves modelingthe plume 82. This may be done for example, using a commerciallyavailable computational fluid dynamics (“CFD”) software package. Theinputs to the software include, but are not limited to, the aerodynamicand thermal characteristics of the shielding gas flow 76 and theaerodynamic and thermal characteristics of the plume generation andpropagation process. The inputs may take into consideration factors suchas: air flow rates, energy beam wavelength, intensity, or focus,consolidated or unconsolidated powdered material composition andphysical characteristics, melt pool dimensions and thermalcharacteristics, the type of fusing process (e.g. heating, melting, orsintering), and the composition of the plume (e.g. gases/and/or metalalloys). The CFD software is then capable of producing as an output theabove-mentioned plume map.

Another possible method for characterizing the plume 82 involves sensingthe plume 82. Any visualization technique capable of distinguishing theplume 82 from the gas flow 76 may be used for this purpose.

For example, an illumination source may be provided to illuminate theplume 82 in concert with one or more sensors to detect light scatteredor reflected from the plume 82. Nonlimiting examples of suitableillumination sources include: a laser operated at a low output wattage(such as the beam generators 24); one or more additional dedicatedlow-power lasers (shown schematically at 85 in FIG. 5), a supplementarylight-emitting diode (“LED”), or a chamber light in an appropriatewavelength (e.g. infrared or visible). Both backscatter and forwardscatter sensing techniques may be used, and multiple images frommultiple sensors may be combined to generate a 3-D plume map.

In the example shown in FIG. 5, an illumination source 86 (shownschematically) is provided at a fixed location within the housing 26.Sensors 88 are provided within the housing 26 with a clear field of viewof the build surface 45. Each sensor 88 is sensitive to forwardscattered light 90 or backward scattered light 92. The sensors 88 are ofa type and configured such that they can produce a signal representativeof the position of the plume 82. For example, they may be imagingsensors, or a plurality of simpler sensors arranged in an X-Y array maybe provided in order to provide positional reference. The pattern ofsignals from the sensors 88 is indicative of the location of the plume82.

The sensors 88 may be used to generate a plume map in real time as thebuild process proceeds. Alternatively, the sensors 88 could be used aspart of an empirical method of characterizing the plume 82. Initially, atest build would be performed using a nominal set of operatingparameters, without any effort to avoid beam-plume interactions. Thesensors 88 would be used to create a plume map as described above. In asecond iteration, the plume map would be used to implement changes inthe build parameters using one or more of the beam-plume avoidancemethods described below. The sensors 88 could be used again to determinethe effectiveness of the changes. A series of iterations may beperformed until the operating parameters result in minimal beam-plumeinteractions. Once this set of iterations is complete, subsequent buildscould be performed in an open loop using the optimized set of operatingparameters.

Using the information provided by one or more of the methods describedabove of characterizing the plume, the machine 10 may be controlled insuch a way as to prevent undesirable interaction between the build beams54, 56 and the plumes 82, 84. Several of these techniques involvecontrolling the build beams 54, 56 with reference to the plume mapsdescribed above.

For example, one possible method involves controlling the operation ofthe beam generators 24 so that the build beams 54, 56 do not interactwith the plumes 82, 84 by dividing the build surface 45 into virtualzones. Referring to FIG. 6, the build surface 45 is virtually dividedinto first and second zones 94, 96 by a virtual boundary 98 extendingparallel to the direction of the gas flow 76 (i.e. parallel to theY-direction). In operation, the build beam 54 is limited to operationwithin the first zone 94 and the build beam 56 is limited to operationwithin the second zone 96. Using this method, it can be seen that theplume 82 from the first build beam 54 would inherently remain clear ofthe second build beam 56 and the plume 84 from the second build beam 56would remain clear of the first build beam 54. Furthermore, each buildbeam 54, 56 would remain clear of its respective plume 82, 84 so long asthe build beam 54, 56 consistently scans in the upstream directionrelative to the gas flow 76.

Another possible method involves controlling the operation of the beamgenerators 24 so that the build beams 54, 56 are diverted away from or“skip over” the plumes 82, 84. Referring to FIG. 7, build beam 54 isshown generating plume 82 and build beam 56 is shown traversing anintended path 100 which would intersect the plume 82. Using this method,the build beam 56 would be momentarily shut off at the point ofpredicted intersection with the plume 82, and then restarted to continuefollowing the intended path 100 on the opposite side of the plume 82 (orpossibly steered in a path completely avoiding the plume 82). Theremaining portion of the path 100 may then be fused at a subsequent timeafter the plume 82 has moved away. Alternatively, the build beam 56could be “skipped” away from its nominal path only when an actualintersection has been detected.

Another possible method involves coordinating the operation of the beamgenerators 24 so that the plume generation is momentarily interruptedproviding a gap for a build beam. Referring to FIG. 8, build beam 54 isshown generating a plume 82 and build beam 56 is shown traversing anintended path 100 which would intersect the plume 82. Using this method,the build beam 54 would be momentarily shut off at a time prior to thepredicted intersection, thus creating a gap 102 in the plume 82. Thetiming and duration of the shut off is chosen such that the plume gapwill allow the build beam 56 to traverse the intended path withoutinterruption or encountering the plume 82.

Any of these techniques may be implemented using a single beam generator24 or multiple beam generators 24.

The operation of the apparatus described above including the machine 10and gas flow apparatus 62 may be controlled, for example, by softwarerunning on one or more processors embodied in one or more devices suchas a programmable logic controller (“PLC”) or a microcomputer (notshown). Such processors may be coupled to the sensors and operatingcomponents, for example, through wired or wireless connections. The sameprocessor or processors may be used to retrieve and analyze sensor data,for statistical analysis, and for feedback control.

The method described herein has several advantages over the prior art.In particular, it enables the use of multiple energy beams orrapidly-scanned energy beams in order to speed up an additivemanufacturing process.

It will improve part quality by maintaining uniform energy beam densityand focus.

The foregoing has described an apparatus and method for plume avoidancein an additive manufacturing process. All of the features disclosed inthis specification (including any accompanying claims, abstract anddrawings), and/or all of the steps of any method or process sodisclosed, may be combined in any combination, except combinations whereat least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying potential points of novelty, abstract and drawings), orto any novel one, or any novel combination, of the steps of any methodor process so disclosed.

What is claimed is:
 1. A method of controlling an additive manufacturingprocess in which one or more energy beams are used to selectively fuse apowder to form a workpiece, in the presence of one or more plumesgenerated by interaction of the one or more energy beams with thepowder, the method comprising: mapping the additive manufacturingprocess with a first set of operating parameters to determine atrajectory of the one or more plumes and to predict intersection of theone or more energy beams with the trajectory of the one or more plumes;modifying the first set of operating parameters to create a modified setof operating parameters when an intersection is predicted; andcontrolling at least one of a trajectory and a timing of the one or moreenergy beams using the modified set of operating parameters so as toprevent the one or more energy beams from intersecting the one or moreplumes.
 2. The method of claim 1 wherein one or more of the energy beamsare steered so as to avoid intersecting the one or more plumes.
 3. Themethod of claim 2 wherein one of the energy beams is steered to avoidone of the plumes generated by the same energy beam.
 4. The method ofclaim 2 wherein one of the energy beams is steered to avoid one of theplumes generated by a different one of the energy beams.
 5. The methodof claim 1 wherein one or more of the energy beams are interrupted tocreate a gap in one or more of the plumes, so that one or more of theenergy beams may pass though the gap.
 6. The method of claim 5 whereinone of the energy beams passes through a gap in one of the plumesgenerated by the same energy beam.
 7. The method of claim 5 wherein oneof the energy beams passes through a gap in one of the plumes generatedby a different one of the energy beams.
 8. The method of claim 1,further comprising determining the trajectory of the one or more plumesby sensing.
 9. The method of claim 1, further comprising determining thetrajectory of the one or more plumes by modeling.
 10. The method ofclaim 1 wherein an electronic controller with access to sensed orpredicted plume trajectories applies this information to determine whento steer or interrupt one or more of the energy beams.
 11. The method ofclaim 1 wherein a path is selected for each of the one or more energybeams prior to beginning the additive manufacturing process.
 12. Amethod of making a workpiece, comprising: performing a test build usinga nominal set of operating parameters; generating a map from the testbuild; determining an existence of at least one intersection of one ormore energy beams and a plume within the map; modifying the nominal setof operating parameters to create an optimized set of operatingparameters when the existence of at least one intersection of the one ormore energy beams and the plume within the map is determined; depositinga powdered material in a build chamber disposed in a housing, whileusing a gas flow apparatus coupled in fluid communication with thehousing to provide a gas flow over the powder; in the presence of thegas flow, directing one or more energy beams to selectively fuse thepowdered material in a pattern corresponding to a cross-sectional layerof the workpiece, wherein interaction of the one or more energy beamswith the powdered material generates one or more plumes entrained in thegas flow; and controlling at least one of a trajectory and a timing ofthe one or more energy beams using the optimized set of operatingparameters, so as to prevent the energy beams from intersecting theplumes.
 13. The method of claim 12 wherein one or more of the energybeams are steered so as to avoid intersecting the one or more plumes.14. The method of claim 13 wherein one of the energy beams is steered toavoid one of the plumes generated by the same energy beam.
 15. Themethod of claim 13 wherein one of the energy beams is steered to avoidone or more of the plumes generated by a different one of the energybeams.
 16. The method of claim 12 wherein one or more of the energybeams are interrupted to create a gap in one or more of the plumes, sothat one or more of the energy beams may pass though the gap.
 17. Themethod of claim 16 wherein one of the energy beams passes through a gapin one of the plumes generated by the same energy beam.
 18. The methodof claim 16 wherein one of the energy beams passes through a gap in oneof the plumes generated by a different one of the energy beams.
 19. Themethod of claim 12, further comprising determining the trajectory of theone or more plumes by sensing.
 20. The method of claim 12, furthercomprising determining the trajectory of one or more of the plumes bymodeling.
 21. The method of claim 12 wherein an electronic controllerwith access to sensed or predicted plume trajectories applies thisinformation to determine when to steer or interrupt one of the one ormore energy beams.
 22. The method of claim 12 wherein a path is selectedfor each of the one or more energy beams prior to beginning the additivemanufacturing process.
 23. The method of claim 12 further comprisingrepeating in a cycle the steps of depositing and fusing to build up theworkpiece in a layer-by layer fashion.